1. Field of the Invention
The present invention relates to an X-ray waveguide, and more specifically, to an X-ray waveguide to be used in an X-ray optical system for an X-ray analysis technology, an X-ray imaging technology, an X-ray exposure technology, or the like.
2. Description of the Related Art
When an electromagnetic wave having a short wavelength of several ten nanometers or less is dealt with, a difference in refractive index for any such electromagnetic wave between different materials is extremely small, specifically, 10−5 or less. Hence, the critical angle for total reflection also becomes extremely smaller. In view of the foregoing, a large-scale spatial optical system has been used for controlling such electromagnetic wave including an X-ray, and has still been in the mainstream now. As main parts for forming the spatial optical system, there is given a multilayer mirror obtained by alternately laminating materials having different refractive indices, and the multilayer mirror is playing various roles such as beam shaping, spot size conversion, and wavelength selection.
A conventional X-ray waveguide such as a polycapillary propagates, in contrast to such spatial optical system, which has been in the mainstream, an X-ray by confining the X-ray in itself. Researches have been recently conducted on an X-ray waveguide, which propagates an X-ray by confining the X-ray in a thin film or a multilayer film, with a view to reducing the size and improving the performance of an optical system. Specifically, researches have been conducted on, for example, a thin-film waveguide having such a shape in which a waveguiding layer is interposed between two layers of one-dimensional periodic structures (Physical Review B, Volume 67, Issue 23, p. 233303 (2003)) and an X-ray waveguide having such a shape in which an X-ray is confined in multiple adjacent waveguide structures by total reflection before the X-ray is guided (Journal Of Applied Physics, Volume 101, Issue 5, p. 054306 (2007)). In addition, it has been proposed that a waveguide structure is formed with a material, which has an artificially changed refractive index, by providing the inside of a semiconductor such as silicon with a random air pore region through an anodization step (Japanese Patent Application Laid-Open No. 2005-258406).
In Japanese Patent Application Laid-Open No. 2005-258406, however, the following material such as a porous silicon is used in a cladding. Random air pores are formed in the material so that the material may have a relatively reduced electron density as compared with that of a waveguiding region (core) for guiding an electromagnetic wave. As a result, the refractive index of the core for an X-ray becomes relatively small as compared with that of the cladding. Therefore, with the configuration, it is difficult to guide the X-ray by confining the X-ray in the core.
In addition, Journal Of Applied Physics, Volume 101, Issue 5, p. 054306 (2007), only total reflection at an interface between a core and a cladding is used as means for confining an X-ray in the core. Accordingly, there arises such a problem that the selectivity of materials is limited and the range of designs narrows. The foregoing is interpreted as described below. As a the critical angle for total reflection at the interface between the core and the cladding depends only on a characteristic of a material, set values for the structure parameters of the waveguide such as a waveguide width must each fall within a narrow range in, for example, the case where a specific mode is to be guided. In addition, when only the total reflection is used, individual structure errors such as instabilities at the time of production present at the interface between the core and the cladding, and further, in the core and the cladding, the unavoidable discontinuity of the interface, and a crack cause serious reductions in propagation characteristics such as a reduction in transmittance and the deterioration of a waveguide mode. Further, when an X-ray is guided by using only the total reflection, the waveguide mode itself also depends only on the material and a structure determined by the material, and hence the mode is hard to control freely. In contrast, the configuration in Physical Review B, Volume 67, Issue 23, p. 233303 (2003) is such that an X-ray is confined in a core by using not only total reflection but also Bragg reflection at a cladding formed of a one-dimensional periodic structure obtained by alternately laminating two kinds of materials having different refractive indices in the direction perpendicular to a surface plane of substrate. However, the one-dimensional periodic structure exerts its effect almost only in the direction perpendicular to a substrate surface, and cannot exert the effect in a direction parallel to the substrate surface. Accordingly, there arises such a problem that it is extremely difficult to control a mode parallel to the surface or waveguiding direction. Waveguiding direction means the guiding direction of X-ray of a waveguide mode.